Vacuum regeneration

Extending partial copper passivation from one and two hydrogen atoms to three, one expects to obtain a fully passivated (Cu,H3) center. Strong support for the formation of such an electrically inactive center stems from hydrogenation experiments that have shown that the concentrations of all copper-related levels are reduced. Prolonged annealing in a vacuum regenerates the various level (Pearton, 1982). 

If the recovery of the adsorbed VOCs is highly desirable, then instead of steam, a vacuum regeneration system may be used. According to this method, the VOCs are forced to volatilize not by temperature, but by means of pressure. Specifically, a vacuum pump is employed to decrease the pressure in the carbon below the vapor pressure of the VOCs, which leads to then1 boiling at ambient temperature. This method is generally used with carbons, polymers, and zeolite adsorbents (EPA, 1999). 

Figures 9 and 10 illustrate changes in two dependent variables dynamic N2 adsorption capacities and CH4/N2 separation factors. Independent variables are column temperature, operating pressure, and time allowed for vacuum regeneration. This experimental series used a constant feed rate of 6.0 1/min over a time of 1.00 min into a 1" dia. x 24" long adsorber filled with 180g of zeolite. Column depressurization took place for 1.00 min. and this was followed by a variable length vacuum regeneration.

Figure 22.1.23. Vapor recovery by adsorption and vacuum regeneration (After reference 39).

W.N. Tuttle, Recent developments in vacuum regenerated activated carbon based hydrocarbon vapour recovery systems. Port Technology International, 1, pp. 143-146,1995. 

In some cases, process modifications can be made to accommodate the compounds listed previously as unsuitable. These include the use of vacuum regeneration to remove high molecular weight compounds from the carbon and the use of a hot inert gas instead of steam to permit the adsorption of readily hydrolyzable compounds. Another approach is to use special adsorbents, such as activated carbon fibers, which are less subject to the listed problems. 

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